The treatment of contaminated in situ soils and groundwater has gained increased attention over the past few years because of the increasing number of uncontrolled hazardous waste disposal sites. It is well documented that the most common means of site remediation has been excavation and landfill disposal. While these procedures remove contaminants, they are extremely costly and in some cases difficult if not impossible to perform.
More recently, research has focused on the conversion of contaminants contained in soil and groundwater based on the development of on-site and in situ treatment technologies. In situ treatment technologies are those that are performed in the natural environment of the contaminated soil/groundwater. These treatment technologies contrast with ex situ treatment technologies which remove the soil and/or groundwater from the natural environment to a controlled environment such as a treatment vessel. While ex situ systems are typically effective because a) they treat relatively small finite batches of soil and/or groundwater and b) they enable uniform distribution of contaminant treating reagents, nonetheless, such systems are disadvantageous because the contaminated environment must be transported to an offsite location at considerable expense.
One such in situ treatment has been the incineration of contaminated soils. The disadvantage of this system is in the possible formation of harmful by-products including polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF). Biological soil treatment and groundwater treatment is another such in situ system that has been reviewed in recent years. So-called bioremediation systems, however, have limited utility for treating waste components that are biorefractory or toxic to microorganisms.
Such in situ bioremediation systems were the first to investigate the practical and efficient injection of hydrogen peroxide into groundwater and/or soils. These investigations revealed that the overriding issue affecting the use of hydrogen peroxide in situ was the instability of the hydrogen peroxide downgradient from the injection point. The presence of minerals and the enzyme catalase in the subsurface catalyzed the disproportionation of hydrogen peroxide near the injection point, with rapid evolution and loss of molecular oxygen, leading to the investigation of stabilizers as well as biological nutrients.
During the early biological studies from the 1980s, some investigators recognized the potential for competing reactions, such as the direct oxidation of the substrate by hydrogen peroxide. Certain researchers also hypothesized that an unwanted in-situ Fenton's-like reaction under native conditions in the soil was reducing yields of oxygen through the production of hydroxyl radicals. Such a mechanism of contaminant reduction in situ was not unexpected, since Fenton's-type systems have been used in ex situ systems to treat soil and groundwater contamination.
Other investigators concomitantly extended the use of Fenton's-type systems to the remediation of in situ soil systems. These studies attempted to correlate variable parameters such as hydrogen peroxide, iron, phosphate, pH, and temperature with the efficiency of remediation.
As with the bioremedial systems, in situ Fenton's systems were often limited by instability of the hydrogen peroxide in situ and by the lack of spatial and temporal control in the formation of the oxidizing agent (hydroxyl radical) from the hydrogen peroxide. In particular, aggressive/violent reactions often occurred at or near the point where the source of the oxidizing agent (the hydrogen peroxide) and the metal catalyst were injected. As a consequence, a significant amount of reagents including the source of the oxidizing agent (hydrogen peroxide) was wasted because activity was confined to a very limited area around the injection point. In addition, these in situ Fenton's systems often required the aggressive adjustment of groundwater pH with acid, which is not desirable in a minimally invasive treatment system. Finally, such systems also resulted in the mineralization of the subsurface, resulting in impermeable soil and groundwater phases due to the deleterious effects of the reagents on the subsurface soils.
Other researchers such as Susan J. Masten “The Use of Ozonation to Degrade Organic Contaminants in Wastewaters” Env. Sci. Technol. Vol. 28, No. 4 (1994) have investigated the use of ozone, either alone or in combination with hydrogen peroxide, in ex situ advanced oxidation processes (AOPs). These systems suffered from a similar limitation as the ex situ Fenton's systems; namely, the necessity to pump contaminants from the in situ media to an external reaction vessel, a requirement which was both expensive and inefficient. Ozonation processes also suffered from low selectivity of contaminant destruction and high instability of the ozone and reactive species generated. In addition, it has been observed that from ex situ applications intermediate to high pH conditions are favored for the effective decomposition of ozone in the presence of hydrogen peroxide. These conditions can result in the acidification of the subsurface or mineralization of soils.
It would be of significant advantage in the art of removing contaminants from soil and/or groundwater to provide a system by which the source of the oxidizing agent can travel from the injection point throughout the aerial extent of the contamination in order to promote efficient destruction of the contaminant plume without the acidification of the subsurface or the resultant mineralization of the soils. It would also be a significant advantage in the art to generate a variety of reactive species in sufficient quantity to allow the efficient degradation of a number of contaminants including traditionally recalcitrant chlorinated solvents such as polychloroethylenes and trichloroethylenes.
It would be a further benefit in the art to provide a system which efficiently generates the hydroxyl radical to provide a cost efficient and effective method of oxidizing contaminants in soil and/or ground water.